Varrentrapp reaction

What is Varrentrapp reaction?

Varrentrapp first reported a drastic alkaline degradation of unsaturated carboxylic acids in 1840. This reaction, known as the Varrentrapp reaction, involves fusing the acid with an alkali to produce saturated carboxylic acids with at least two fewer carbon atoms, acetic acid, and hydrogen.

Varrentrapp reaction - general reaction scheme
Varrentrapp reaction

The reaction time and product yields depend on the location of the original double bond in the acid and the length of the hydrocarbon chain. The double bond migrates to the α,β-position of the carboxyl group, and the α,β-unsaturated carboxylic acid undergoes fission in a manner of retro-aldol or retro-Claisen condensation, thus facilitating the movement of the double bond away from the carboxyl group prolongs the reaction time.

The Varrentrapp reaction also causes the migration of the carbon-carbon triple bond, probably involving a reversible acetylene-allene rearrangement. Simple unsaturated carboxylic acids such as acrylic acid, crotonic acid, cinnamic acid, and 2-phenylacrylic acid also undergo similar reactions.

The Varrentrapp reaction of oxygenated carboxylic acids is relatively complex, involving the dehydrogenation of the primary alcoholic moiety to a carboxyl group and the conversion of the secondary alcoholic moiety into a carbonyl group. The resulting dicarboxylic acid or keto acid may undergo various reactions. The saturated epoxy carboxylic acid also undergoes degradation, involving at least three different routes: hydrolysis, reduction or rearrangement, and β-elimination. The trans-epoxide undergoes primarily the β-elimination route, whereas cis-epoxide takes either of the other two paths. An alkoxy moiety also undergoes a β-elimination route during the Varrentrapp reaction.

Some unsaturated carboxylic acids also undergo polymerization instead of fragmentation, as demonstrated by the exclusive polymerization of 6,12-tridecadiynoic acid when fused with alkali at 300 ºC for an hour.

While Varrentrapp reaction has applications in organic synthesis, its greatest value lies in the industrial production of specific carboxylic acids from readily available unsaturated fatty acids.

References

Varrentrapp, F., Ann., 1840, 35, 196

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